1. Technical Field
This invention relates to electrical test systems, and more particularly to a test system incorporating an isolation network. The isolation network is adapted to be coupled between a device under test (DUT) and a DUT support system for generating an impedance which causes the DUT to emit its highest noise voltage levels during a test. This assists in developing worst case interference parameters which provide a guide for modifying the test item's components and connectors to minimize the effects of electromagnetic interference (EMI) generating sources and to immunize potential EMI receptors within the DUT.
2. Discussion
Various techniques have been employed to detect, measure and then suppress EMI in sensitive electrical equipment or test items. Interference or susceptibility detection and measuring should be conducted with the test item operating as close to service conditions as possible. Also, the test item normally is operated in its intended manner with anticipated inputs applied and its outputs normally loaded.
A problem exists with simulating service conditions in a normal manner of operation. To bring a test item out of its normal operating environment and to place it on a bench for a test, an actual duplication of operation of the test item seldom occurs. For example, if in a vehicle equipped with an engine control system that includes a microcomputer as a controller, assume it is desired to use a particular portable mobile two-way radio. Discovering that the radio works when the engine is shut off but doesn't work well while the engine is running, the engine control system is then removed from the vehicle and placed on a test bench for study. A simulator (support for the engine control system) is used to make the control system work as if it is in the vehicle. Also, assume the system responds as if it is operating in the vehicle controlling what it is supposed to be controlling. Assume also, the engine control system generates the same interference that it was putting out before it was placed on the bench, but now the interference reacts with the simulator. By virtue of changes in wiring, the coupling between harnesses, other components, and the impedance and length of wires, the engine control system generally radiates and conducts a different amount of interference.
Efforts have been made to standardize bench test setups in order to gain data that approaches actual circumstances. In prior bench tests under similar circumstances, line impedance stabilization networks (LISNs) have been recommended in a number of interference and susceptibility specifications for insertion in power leads to offer something approaching a standard impedance to the radio frequency (RF) current from test items. The LISN's, as required by some military specifications, introduce a standard 50 ohm power-source impedance for the test item so that conducted RF interference measurements can be compared to pass/fail limits without accounting for a source impedance variable. However, in several LISN designs a 5-microhenry coil is used so the device is suitable for use from 150 KHz to 25 MHz. Over this range, the source impedance varies from about 5 ohms at 150 KHz to 50 ohms at 25 MHz. It is not usable much above 25 MHz due to stray impedance. While it does furnish a standard impedance, it is not the impedance seen in the normal installation. It was never intended to be anything other than an A.C. power lines simulator.
Normally when trying to identify the potential of a device to be an interference source, it was thought that this determination depends upon how interference emanating from the source was measured. This implies that different test processes produced different results for the same interference source. Realizing the above conditions exist, efforts were made toward devising an interference measuring technique that didn't depend upon how the interference was measured. Systems which have been previously developed for this purpose are disclosed in U.S. Pat. Nos. 4,763,062 and 5,541,521, both assigned to the assignee of the present application, and hereby incorporated by reference into the present application. These test systems have performed in an exemplary fashion but nevertheless have been limited to devices under test which have one input referenced to ground.
With devices under test which do not have one signal line referenced to ground (i.e., balanced devices such as devices having differential inputs), previously developed EMI test systems have not been suitable for intercoupling between the device under test support equipment and the device under test itself. Accordingly, it is a principal object of the present invention to provide a test system forming an isolation network which is suitable to be intercoupled between a device under test and a device under test support system which is capable of providing a high isolation impedance to common mode signals present at input terminals of the isolation network, while still providing a relatively lossless path for differential signals present across the input terminals.
It is a further object of the present invention to provide an isolation network incorporating a bifilar winding which is relatively inexpensive to produce, which provides a broad bandwidth, and which enables balanced devices to be tested under worst case EMI operating conditions such that the device under test is forced to produce its highest noise voltage.
A test system as described above would therefore enable worst case interference parameters to be established and would also provide a repeatable scheme for determining worse case signals in any environment in which the device under test is employed. After establishing the worse case interference parameters, modification techniques and/or test item circuit components and connectors may be employed to minimize the effect of generating EMI sources and/or to immunize EMI susceptible receptors within the device under test.